2.1 Study Design
The prospective diagnostic study was conducted in the department of anesthesiology of People's Hospital of Peking University and was approved by the Institutional Review Board of our institution (Ethics Committee of Peking university people's hospital 2020PHB139-01). The study was registered at the Chinese clinical trial registry (ChiCTR2000034940). Written informed consents were obtained from all participants. Patients who underwent general anesthesia with tracheal intubation for abdominal surgery were consecutively included. Inclusion criteria were: age at 18–70 years old, ASAI-III, NYHA I-II grade. Exclusion criteria included TEE contradictions such as gastroduodenal ulcer, the history of esophagus operation, esophagus fundus ventricular varication, and arrhythmia, left ventricle EF < 55%, average E/e’ >14 or e’ average < 9cm/s at baseline, severe valvular disease.
Upon arrival in the operating room, all patients were monitored with pulse oximetry and electrocardiograph, radial artery catheterization connecting the FloTrac™/Vigileo™ ( Edwards Lifesciences,USA). Anesthesia induction was done with midazolam(1mg), sufentanil (0.25µg/kg), etomidate (0.3mg/kg), rocuronium (0.6-1mg/kg) and maintained with sevoflurane inhalation. All patients were continuous monitored with BIS (range of 40–60). Patients were mechanically ventilated in volume-controlled ventilation: VT of 8ml/kg, respiratory rate of 12 breath/min, no PEEP was applied.
After induction, the TEE probe (6TC-RS GE Medical Horton,Norway)was inserted orally. During the whole measurement, the patients’ position remained unchanged.
2.2 Data collection
Group 1 basic hemodynamic data: MAP, HR, and CVP;
Group 2 functional hemodynamic data using FloTrac: CI and SVV;
Group 3 echocardiographic data using TEE: SVC-CI, E velocity, E/e’, LVEDd (left ventricular end-diastole diameter).
SVC-CI (via M-mode), E velocity (via pulse wave doppler), e’ (e’= e’lateral + e’ septal /2, via tissue doppler from both lateral and septal side of mitral annular), LVEDd (via M-mode).
2.3 Fluid responsiveness
Fluid challenge: A fluid challenge was conducted with 10ml/kg of a Ringer's lactate for 15–20 minutes.
△CI was calculated as follows: △CI=(CI after -CI baseline)/ CI baseline *100%. Patients were classified as Responders (FR group: △CI ≥ 15%) and Non-Responders (NR group:△CI༜15%).
We also calculated the △LVEDd using the formula analogous to △CI:
△LVEDd = (LVEDdafter- LVEDdbaseline)/LVIDdbaseline*100%
2.4 Data measurements
2.4.1 SVC-CI measurement
After tracheal intubation, we passed the TEE probe into the mid-esophagus (ME) position. The transducer angle was rotated forward from 90° to 110°, to obtain the ME Bicaval View. In this view, the superior vena cava (SVC) and the right atrium (RA) can be well imaged. The SVC diameter was measured approximately 2 cm from the junction with RA using the M-mode. Take the M-mode cursor to the junction and perpendicular to the SVC and obtain the inner diameter within a single respiratory cycle (Fig. 1) [15]. Measure the maximum and minimum diameter over a single respiratory cycle. SVC-CI was calculated as follows: SVC-CI= (SVCmax − SVCmin)/SVCmax*100%. Echocardiographic variables were derived from the US machine (Vivid 7 Pro, GE Vingmed Ultrasound AS, Horten, Norway). All measurements were made three times and the average was used for statistical analysis.
2.4.2 LVEDd measurement
In the trans-gastric left ventricular (LV) short-axis view at the midpapillary level, LV inner diameter could be measured using M-mode imaging [16].
2.4.3 E velocity and E/e’ measurement
At the ME four-chamber view, position the pulse wave (PW) Doppler sample volume between mitral leaflet tips, and adjust the sample volume to align with the blood flow, then obtain the optimal image of the E wave. At the same view, position the tissue Doppler (TDI) sample volume both at lateral and septal basal regions of mitral annular to acquire e’lateral and e’septal. The average e’ velocity can be computed: e’average= (e’septal + e’lateral)/2 [17].
All measurements were performed by a National Board of qualified Echocardiography anesthesiologist strictly following the relevant guidelines [15–18].
2.5 Statistical analysis
For continuous variables, data were expressed as mean ± SD (normality distribution) or median with interquartile range (non-normality distribution). For categorical variables, percentages were calculated and the normality distribution was assessed by the Shapiro-Wilk normality tests, and comparisons of percentages were performed with Fisher’s exact test. The differences between Responders and Non-Responders were assessed using the Mann-Whitney U-test or Student’s t-test. Spearman’s rank method was performed to test linear correlations between △LVEDd and △CI, basic SVC-CI, and △CI.
To determine the ability to predict fluid responsiveness, Receiver operating characteristics (ROC) curves were generated and the area under the ROC curve (AUROC) was calculated. All P -values were two-tailed and a P-value < 0.05 was considered significant. All statistical analyses were performed with IBM SPSS Statistics 26.0 (IBM, Somers, NY, USA)
2.6 Sample size
Medcalc software (Windows 19.4, Ostend, Belgium) was used to calculate the sample size. According to the pilot study, we assume the AUROC of SVC-CI was 0.75, with an α error of 0.05 and power of 0.9, and the ratio of sample size in the FR/NR group of 1. 26 patients were required for each group. Considering dropout, we planned to recruit 60 patients finally.